There's nothing unrealistic about restoring the Roman empire. It didn't happen, but it would have been a plausible. Note that realism doesn't mean that everything has to happen exactly as it did in History.
Exactly the point, though the other way around. Restoring the roman empire at the charlemagen start date or later is compeltely and utterly implausible. The Byzantines were in no position to retake Syria, much less anything else.
Why?
Because the ability to muster troops and raise taxes does
not grow linearly with size. The Byzantines or indeed any empire had
more problems to muster armies than medium sized kingdoms.
In-game, there really are only 2 options how to slow or prevent this nonsensical super-blobs that can raise armies
thousands of miles apart on the same day. The devs could have either a) implemented
massive penalties for levies and taxes scaling with distance to the capital or b) introduce pacts, coalitions or whatever you want to call it and revitalize the idea of collective security which is, by the way, a prevalent general idea since the early stone age.
Considering the outcry option A would have caused by the "aww blobbing is now too hard for us byzantophiles" faction here, the devs chose the (perceived) lesser evil.
EDIT: By the way, before accusing me of justifying all actions of Pdx, look at some of my older postings. The way is critizised Pdx in harsh, even isulting, terms for not fixing seduction, super stability or this pointless blobbing (not to mention sunset invasion) was quite, well, vocal. Concalve made CK2 worth playing again for me, therefore i´m a bit miffed about these recurring threads.